Fix error highlighting of closed subgraphs in Bridges.

Fix error highlighting of closed subgraphs in Bridges.

When I did the recent rewrite of the Bridges drawing code, I spotted
and replicated the code in game_redraw that set G_WARN at redraw time
on an island with unsatisfiable clues, but didn't spot the code
elsewhere (e.g. map_group_check) which might have set G_WARN ahead of
time, so I failed to check for that. As a result, if you join some
islands together into a subgraph with no further room for expansion
(e.g. connect a pair of 1s together, or a pair of 2s with a double
bridge) then all the bridges in that subgraph light up red but the
islands themselves forget to.

Trivial one-line fix.